Renewable self-healing capsule system

ABSTRACT

A renewable material for releasing a self-healing agent includes a renewable polymeric substrate with capsules and a reactant dispersed in the renewable polymeric substrate. The capsules may be formed from a first renewable shell polymer and may enclose the renewable self-healing agent. The reactant may be suitable for reacting with the renewable self-healing agent to form a polymer.

TECHNICAL FIELD

This invention relates to the field of self-healing materials. Moreparticularly, it relates to renewable self-healing agents encapsulatedby a renewable polymer.

BACKGROUND

Polymeric materials are used in many applications, including paints,upholstery, pipes, and circuit boards. Polymeric materials can undergodegradation due to a number of factors, including heat, chemicals, andmechanical forces.

SUMMARY

In one embodiment, a renewable self-healing material includes arenewable polymeric substrate with capsules and a reactant dispersed inthe renewable polymeric substrate. The capsules may be formed from afirst renewable shell polymer and enclose the renewable self-healingagent. The reactant may be suitable for reacting with the renewableself-healing agent to form a polymer.

In another embodiment, a method for creating a renewable self-healingmaterial includes creating a microemulsion having a continuous phase anda first dispersed phase, wherein the continuous phase includes a firstrenewable shell polymer and a first solvent, and the first dispersedphase includes a renewable self-healing agent. The solubility of thefirst renewable shell polymer in the continuous phase is decreased toform a second dispersed phase, wherein the second dispersed phase has ahigher concentration of the first shell polymer than the continuousphase. A capsule is formed around the first dispersed phase, wherein thecapsule contains the first renewable shell polymer. The capsule and areactant are dispersed into a renewable polymeric substrate, wherein thereactant is suitable for reacting with the renewable self-healing agentto form a polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example, and not by way oflimitation, in the figures of the accompanying drawings in whichreference numerals refer to similar elements.

FIG. 1 represents a two dimensional cross-sectional representation of acrack in a polymeric material having reactant and renewable capsulesthat contain a renewable self-healing agent, according to embodiments ofthe invention.

FIG. 2 depicts capsule formation through coacervation, according toembodiments of the invention.

DETAILED DESCRIPTION

Polymeric materials can undergo degradation due to a number of factors,including heat, chemicals, and mechanical forces. One result of polymerdegradation is cracking of the polymer. Cracking may occur throughoutthe polymeric material, both on the outside surface where the cracks maybe visually detected, and on the inside surface where cracks may gounseen. These cracks can lead to equipment failure in circuit boards,fluid piping systems, and other applications with polymeric materials.One solution to this cracking is to use self-healing capsules in thepolymer that may autonomically heal the polymer upon crack formation andcapsule rupture.

Many plastics and polymers are derived from petroleum sources. Industryhas been moving away from petroleum products, and many polymers such aspolylatic acid (PLA) are created from renewable sources. It may bedesired for marketing or “green energy” requirements for these renewablepolymers, or “biopolymers,” to contain renewable materials throughoutthe polymer. Conventional self-healing materials do not use renewableself-healing mechanisms for biopolymers.

According to embodiments of the invention, a biopolymer may containrenewable self-healing capsules which may assist in healing thebiopolymer upon a crack. Renewable materials are any materials that maybe generated from biological or natural processes, and the term“renewable” refers to the property of the item discussed as having beengenerated through biological or natural processes. The renewableself-healing capsules may include a polymer shell encapsulating theself-healing agent made from renewable materials. The renewableself-healing capsules may be formed through coacervation.

Self-Healing Mechanism

According to embodiments of the invention, the mechanism for a renewableself-healing material involves a self-healing agent and a catalystcapable of initiating polymerization of the self-healing agent. Theself-healing agent may be a renewable monomer or polymer enclosed in arenewable capsule that may react with a reactant to form a polymericnetwork that fills the crack. The reactant and the capsules containingthe renewable self-healing agent are dispersed in a polymeric material.When a crack forms in the polymeric material, the crack may rupture acapsule, causing the self-healing agent contained in the capsule to flowinto the crack. After the self-healing agent flows into the crack, itmay contact the reactant and form a polymeric network, filling thecrack.

FIG. 1 represents a two dimensional cross-sectional representation of acrack in a polymeric material having a reactant and capsules thatcontain a renewable self-healing agent, according to embodiments of theinvention. Capsules 103 containing a renewable self-healing agent andreactant particles 102 are dispersed into a polymer 101. When a crack105 forms, it may create a capsule rupture 104, causing the renewableself-healing agent to flow into the crack 105. The self-healing agentmay react with a reactant particle 102, polymerize, and seal the crack105.

The self-healing mechanism works with a wide variety of reactants andrenewable monomers and polymers. Renewable self-healing agents may bechosen for their chemical and physical properties, such as low meltingpoint or viscosity, or the surrounding polymeric material's properties,such as residual functionality, depending on the intended applicationand material conditions. Renewable self-healing agents that may be usedinclude, but are not limited to, triglycerides, functionalized vegetableoils such as epoxidized soybean oil, corn oil, and linseed oil. Therenewable self-healing agents may have functional groups attached sothat they more easily polymerize or create a stronger polymeric network.Functional groups that may be used include, but are not limited to,epoxies, acrylates, and hydroxyls. Reactants may be selected for theirability to react with the renewable self-healing agent and form apolymer. The reactants that may be used include, but are not limited to,amines, citric acid, and Lewis acids. In an alternative embodiment ofthe invention, at least some of the capsules contain a reactant whichmay rupture due to crack formation, with the reactants encapsulatedthrough the same method as discussed below.

In an embodiment of the invention, the renewable self-healing agent isepoxidized soybean oil and the reactant is an amine, such as a primaryamine. Soybean oils are high production commercial oils, as soybeans arethe second most produced crop in the United States. Soybean oil ishighly unsaturated, which makes them more easily functionalized. Theepoxidized soybean oil may act as an epoxy resin that reacts with anamine to form a polymeric network. Amine-curing of epoxies is athermosetting reaction that results in a resistive polymer and may occurunder a variety of conditions, making a system of amine-cured epoxidizedsoybean oil ideal for renewable polymers used in most environments. Thereaction between epoxidized soybean oil and an amine may be as follows:

where R and R′ are parts of the triglyceride chains of the epoxidizedsoybean oil and R″ is a side group of a primary amine.

Capsule Structure and Formation

According to embodiments of the invention, the self-healing capsules maybe formed through coacervation. Coacervation involves deposition of acolloid at the interface of a dispersed phase and a continuous phase toform a microcapsule. A self-healing agent may be dispersed into apolymer solution and emulsified to form a microemulsion having adispersed phase of self-healing agent droplets and a continuous phase ofthe polymer solution. The emulsification may be achieved throughagitation, sonication, or other mechanical method. The polymer solutionmay contain a renewable shell polymer in a solvent such as water, wherethe renewable shell polymer may form part of the capsule wall of theself-healing capsules. Renewable shell polymers that may be usedinclude, but are not limited to, collagen, starch, gum arabic, gelatin,dextrin, cellulose, and casein. More than one shell polymer may be usedto form the capsule wall, such as starch-gum arabic.

The solubility of the shell polymer in the continuous phase is reducedthrough a solubility reduction event and a large part of the shellpolymer separates from solution to form a new polymer-rich phase. Thesolubility reduction event may involve addition of an incompatiblepolymer (complex coacervation), non-solvent, or acid to the polymersolution, a temperature or pH change, or a combination of these events.The polymer-rich phase may coat the self-healing agent and depositaround the self-healing agent of the dispersed phase as a continuousshell. Deposition may occur when the polymer is adsorbed at theinterface between the dispersed phase of self-healing agent andcontinuous phase of the polymer-rich phase. The polymer coating may bereinforced through the addition of a renewable cross-linking agent.Renewable cross-linking agents that may be used include, but are notlimited to, gluteraldehyde and transglutaminase.

FIG. 2 depicts capsule formation through coacervation, according toembodiments of the invention. A mixture 200 contains a layer ofself-healing agent 201 and a layer of polymer solution 202. The polymersolution 202 contains a shell polymer in a solvent. The self-healingagent 201 and the polymer solution 202 are emulsified to form adispersed phase of self-healing agent 203 and a continuous phase ofpolymer solution 204. A reduction event reduces the solubility of theshell polymer in the continuous phase of polymer solution 204 and causesit to form a polymer-rich phase 205 and a solvent-rich phase 206. Thepolymer-rich phase 205 coalesces and deposits around the dispersed phaseof self-healing agent 204 to form a polymer coating 207. This polymercoating 207 may be further cross-linked through the addition of across-linking agent.

In an embodiment of the invention, the renewable shell polymers are gumarabic and starch, the self-healing agent is epoxidized soybean oil, andthe solvent is water. The capsule wall is formed through complexcoacervation, in which the solubility reduction event involves additionof an immiscible, oppositely charged polymer such as gelatin, andreduction of pH through addition of a weak acid. After addition of theoppositely-charged polymer, the gum arabic and starch coalesce and forma coating around the epoxidized soybean oil. The renewable shellpolymers of the coating are further cross-linked with gluteraldehyde.The capsules may be washed and separated for dispersion into abiopolymer.

The self-healing capsules may be controlled for size, both for thecapsule and the capsule shell. The thickness of the capsule shell may becontrolled by the concentration of the renewable shell polymers, thetemperature of coacervation, and length of time the coacervation andsettling out of the polymer-rich phase is allowed to continue, as wellas other general factors dictating physico-chemical processes. It may bedesirable to have a thinner capsule wall, depending on the properties ofthe polymeric substrate into which the self-healing capsule is dispersedor the application of the polymeric substrate. For example, the capsulesmay be from 2-1000 microns thick, depending on factors such as theproperties and application of the surrounding polymeric material.

Renewable Polymeric Substrate

Once the self-healing capsules are formed, they may be dispersed into arenewable polymeric substrate. The renewable self-healing capsules maybe incorporated into a variety of renewable polymeric substrates. Thepolymeric substrate may be formed from renewable monomers and polymerswhich include, but are not limited to, sugars such as starch, vegetableoils such as soybean oil, lignin, cellulose, suberin, terpenes, tannins,furans, and acid monomers such as citric acid and tartaric acid. Thepolymeric substrates may be formed by any suitable method, includingstep growth polymerization and chain growth polymerization such ascationic polymerization.

The self-healing capsules may be dispersed into the polymeric substratethrough any suitable method of dispersion and polymer formation, such ascolloidal dispersions. In an embodiment of the invention, the renewableself-healing capsules are dispersed into a solution of renewablemonomers, after which polymerization of the renewable monomer solutionis initiated. The reactant may be dispersed into the polymeric substratein a variety of ways, including capsules, particles, or bound to thepolymeric substrate. The reactant may be encapsulated through the samemechanism as the self-healing agent. The amount of capsules isempirically determined based on the rheology of the polymeric substrate,the capsule size, and the amount of self-healing agent and reactantneeded to reach the desired self-healing probability.

Experimental Protocols

The following illustrative experimental protocols are prophetic exampleswhich may be practiced in a laboratory environment.

Formation of Self-Healing Capsule Through Coacervation; EpoxidizedSoybean Oil, Gum Arabic/Starch/Gelatin Shell

A first aqueous solution containing 2-10 wt % gum arabic and 2-10 wt %starch is stirred for 15 minutes at 40° C. A second aqueous solutioncontaining 10-20 wt % gelatin is stirred for 15 minutes at 40° C. Thefirst aqueous solution is emulsified with epoxidized soybean oil bymechanical agitation for 5 minutes. The second aqueous solution is addedto the agitated emulsion and the pH is lowered to 4 using 1M citricacid. The resulting solution is cooled to 5° C. for 30 minutes withcontinuous mechanical agitation. The pH is raised to 9 under continuousagitation to form the encapsulated epoxidized soybean particles. Theparticles are concentrated by centrifugation, decanted, and spray dried.

What is claimed is:
 1. A method for creating a renewable self-healingmaterial, comprising: creating a microemulsion having a continuous phaseand a first dispersed phase, wherein the continuous phase includes afirst renewable shell polymer and a first solvent, and the firstdispersed phase includes a renewable self-healing agent; decreasing thesolubility of the first renewable shell polymer in the continuous phaseto form a second dispersed phase, wherein the second dispersed phase hasa higher concentration of the first shell polymer than the continuousphase; forming a capsule around the first dispersed phase ofself-healing agent, wherein the capsule contains the first renewableshell polymer; and dispersing the capsule and a reactant into arenewable polymeric substrate, wherein the reactant is suitable forreacting with the renewable self-healing agent to form a polymer.
 2. Themethod of claim 1, wherein the first renewable shell polymer is from thegroup consisting of collagen, starch, gum arabic, gelatin, dextrin,cellulose, and casein.
 3. The method of claim 1, wherein the renewablepolymeric substrate is from the group consisting of sugars, vegetableoils, lignin, cellulose, suberin, citric acid, and tartaric acid.
 4. Themethod of claim 1, wherein the reactant is an amine or a Lewis acid. 5.The method of claim 1, wherein decreasing the solubility involves anaction from the group consisting of changing the pH of themicroemulsion, changing the temperature of the microemulsion, adding areduction polymer with a charge opposite of the first shell polymer tothe microemulsion, and adding a non-solvent to the microemulsion.
 6. Themethod of claim 1, wherein the renewable self-healing agent is afunctionalized triglyceride vegetable oil having a functional group,wherein: the functionalized triglyceride vegetable oil is from the groupconsisting of soybean oil, corn oil, and linseed oil; and the functionalgroup is from the group consisting of epoxy, acrylate, and hydroxyl. 7.The method of claim 6, wherein the renewable self-healing agent isepoxidized soybean oil and the reactant is an amine.
 8. The method ofclaim 1, wherein forming a capsule around the first dispersed phase ofself-healing agent further comprises depositing the first renewableshell polymer onto the renewable self-healing agent in the firstdispersed phase.
 9. The method of claim 8, wherein forming a capsulearound the first dispersed phase of self-healing agent furthercomprising cross-linking the first renewable shell polymer with across-linking agent.
 10. The method of claim 1, further comprising:creating the microemulsion wherein the continuous phase further includessecond renewable shell polymers; decreasing the solubility of the secondrenewable shell polymer to form part of the second dispersed phase,wherein the second dispersed phase has a higher concentration of thesecond renewable shell polymer than the continuous phase; and formingthe capsule around the first dispersed phase of self-healing agent,wherein the capsule further contains the second renewable shell polymer.11. The method of claim 10, wherein: the first renewable shell polymeris starch; the second renewable shell polymer is gum arabic; therenewable self-healing agent is epoxidized soybean oil; the reactant isan amine; forming the capsule around the first dispersed phase furthercomprising cross-linking the first renewable shell polymer and thesecond renewable shell polymer with gluteraldehyde; and decreasing thesolubility of the first renewable shell polymer and the second renewableshell polymer involves adding a gelatin to the microemulsion.